Linear response breakdown in solvation dynamics induced by atomic electron-transfer reactions

The linear response (LR) approximation, which predicts identical relaxation rates from all nonequilibrium initial conditions that relax to the same equilibrium state, underlies dominant models of how solvation influences chemical reactivity. We experimentally tested the validity of LR for the solvation that accompanies partial electron transfer to and from a monatomic solute in solution. We photochemically prepared the species with stoichiometry Na0 in liquid tetrahydrofuran by both adding an electron to Na+ and removing an electron from Na-. Because atoms lack nuclear degrees of freedom, ultrafast changes in the Na0 absorption spectrum reflected the solvation that began from our two initial nonequilibrium conditions. We found that the solvation of Na0 occurs more rapidly from Na+ than Na-, constituting a breakdown of LR. This indicates that Marcus theory would fail to describe electron-transfer processes for this and related chemical systems.